1. Field of the Invention
The present invention relates to a magnetoresistive effect element for generating so-called MR (magnetoresistive) effect in which a resistance value changes with application of a magnetic field from the outside and a magnetic memory device fabricated as a memory device capable of storing information by the use of a magnetoresistive effect element.
2. Description of the Related Art
In recent years, as information communication devices, in particular, personal small information communication devices such as portable terminal devices (e.g. personal digital assistants) are widely spreading, it is requested that devices such as memories and logic devices comprising these information communication devices or portable terminal devices should become higher in performance, such as they should become higher in integration degree, they can operate at higher speed and they can consume lesser electric power. Particularly, technologies that can make nonvolatile memories become higher in density and larger in storage capacity are becoming more and more important as complementary technologies for replacing hard disk devices and optical disk devices with nonvolatile memories because it is essentially difficult to miniaturize hard disk devices and optical disk devices because they cannot remove their movable portions (e.g. head seek mechanism and disk rotation mechanism).
Flash memories using semiconductors and an FeRAM (ferro electric random-access memory) using a ferro dielectric material are widely known as nonvolatile memories. However, flash memories are slow in information write speed as compared with a DRAM (dynamic random-access memory) and a SRAM (static random-access memory). Further, it has been pointed out that the FeRAM cannot be rewritten so many times. Accordingly, a magnetic memory device called an MRAM (magnetic random-access memory) utilizing a magnetoresistive effect has been proposed and receives a remarkable attention as a nonvolatile memory that can overcome these defects (e.g. xe2x80x9cNaji et. al ISSCC2001xe2x80x9d).
The MRAM is able to record information by the use of a giant magnetoresistive effect (giant magnetoresistive: GMR) type storage element or a tunnel magnetoresistive effect (tunnel magnetoresistive: TMR) type storage element (these elements will be generally referred to as a xe2x80x9cmagnetoresistive effect elementxe2x80x9d). The magnetoresistive effect element includes a multilayer film structure including two ferromagnetic material layers and a nonmagnetic material layer made of an insulating material layer or a conductor sandwiched between the two ferromagnetic material layers. One ferromagnetic material layer is used as a free layer (free layer) whose magnetization direction can be inverted and the other ferromagnetic material layer is used as a fixed layer (pinned layer) whose magnetization direction is fixed (pinned) This magnetoresistive effect element is able to record information by utilizing the fact that a resistance value is changed in response to the magnetization direction of the free layer to discriminate xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d of information.
In the MRAM, these magnetoresistive effect elements are arrayed in an XY matrix fashion. The MRAM includes word lines and bit lines crossing these element groups in the horizontal and vertical directions. Then, the magnetization direction of the free layer in the magnetoresistive effect element located at the crossing area is controlled by using a synthetic current magnetic field generated when a current flows through both of the word lines and the bits lines, whereby information can be written in the magnetoresistive effect element. At that time, the magnetization direction of the free layer in each magnetoresistive effect element is not changed by magnetic fields solely generated from the word lines or the bit lines but the above magnetization direction is changed by a synthesized magnetic field of both of the word lines and the bit lines. Accordingly, even when the magnetoresistive effect elements are arrayed in a matrix fashion in the MRAM, information can be selectively written in a desired magnetoresistive effect element.
On the other hand, when information is read out from each magnetoresistive effect element, the magnetoresistive effect element is selected by using a device such as a transistor and the magnetization direction of the free layer in the magnetoresistive effect element is obtained as a voltage signal through MR effect to thereby read out information from the magnetoresistive effect element. This point will be described below more in detail. In general, electron spins are polarized in the ferromagnetic material layer such as the free layer or the fixed layer and up-spins and down-spins become either majority spins having large state density or minority spins having small state density. When the magnetization directions of the free layer and the fixed layer are parallel to each other, if the up-spins in the free layer are majority spins, then up-spins are majority spins in the fixed layer. When on the other hand the magnetization directions of the free layer and the fixed layer are anti-parallel to each other, if the up-spins are majority spins in the free layer, then up-spins in the fixed layer become minority spins. When electrons pass the nonmagnetic material layer between the free layer and the fixed layer, spins are preserved and a probability at which a certain spin will pass the nonmagnetic material layer is proportional to a product of state densities of spins of the two ferromagnetic material layers which sandwich the nonmagnetic material layer. Therefore, when the magnetization directions of the free layer and the fixed layer are parallel to each other, majority spins having large state densities become able to pass the nonmagnetic material layer. When the magnetization directions of the free layer and the fixed layer are anti-parallel to each other, majority spins having large state density become unable to pass the nonmagnetic material layer. For this reason, when the magnetization directions of the free layer and the fixed layer are anti-parallel to each other, a resistance increases as compared with the case in which the magnetization directions of the free layer and the fixed layer are parallel to each other. Therefore, if a voltage between the free layer and the fixed layer is detected through the word lines and the bit lines, then it becomes possible to read out information from the free layer.
As described above, since the MRAM using the magnetoresistive effect element utilizes the magnetization direction of the free layer in the magnetoresistive effect element to judge information, the MRAM is able to record information in a nonvolatile fashion with excellent response characteristics. Further, since the structure of the storage element (memory cell) that can hold information is simple, the magnetoresistive effect element becomes suitable for microminiaturization and increasing integration degree.
However, in the magnetoresistive effect element for use with the above-mentioned related-art MRAM, since the structure of the magnetoresistive effect element is simple, the magnetoresistive effect element is suitable for microminiaturization and increasing the integration degree. However, if the magnetoresistive effect element is microminiaturized much more and is increased in integration degree much more, then disorder of magnetization occurs at the end portion of the magnetoresistive effect element, which causes the following problems to arise.
These problems will be described in detail. Specifically, since the magnetoresistive effect elements are arrayed in the MRAM in a matrix fashion, if the magnetoresistive effect element is microminiaturized much more and is increased in integration degree much more, then each magnetoresistive effect element is influenced by a leakage magnetic field from the adjacent magnetoresistive effect element. There is a risk that coercive force in the free layer of each magnetoresistive effect element will change. The change of such coercive force makes it difficult to select elements when information is written in the MRAM. In particular, the change of the coercive force becomes serious as the element size is reduced in accordance with the microminiaturization and the increase of the integration degree of the magnetoresistive effect element.
In general, within the free layer of the magnetoresistive effect element, a microscopic magnetic moment is not uniform and can take various states such as eddy state, C-like state and S-like state so as to minimize magnetostatic energy. Coercive forces in these respective states are not always the same even when the free layers are made of the same ferromagnetic material. Although the state that the microscopic magnetic moment can take depends upon the shape and size of the magnetoresistive effect element, the shapes used as related-art magnetoresistive effect elements are strip-like shape or elliptic in most cases, there is a possibility that the microscopic magnetic moment will take a plurality of states near, in particular, the end portion of the element. A plurality of such states is not preferable because it causes a coercive force in the free layer of each magnetoresistive effect element to be dispersed.
In view of the aforesaid aspect, it is an object of the present invention to provide a magnetoresistive effect element and a magnetic memory device in which changes and dispersions of a coercive force can be suppressed as much as possible and in which satisfactory information recording characteristics can be realized even when a magnetoresistive effect element and a magnetic memory device are further microminiaturized and are increased in integration degree.
According to an aspect of the present invention, there is provided a magnetoresistive effect element in which at least a free layer made of a ferromagnetic material, a nonmagnetic layer made of a nonmagnetic material and a fixed layer made of a ferromagnetic material and of which the magnetization direction is fixed are laminated in that order and in which information is recorded by the use of a change of the magnetization direction of the free layer. This magnetoresistive effect element includes a magnetic field returning structure for returning a magnetic field generated by the free layer.
According to other aspect of the present invention, there is provided a magnetoresistive effect element in which at least a free layer made of a ferromagnetic material, a nonmagnetic layer made of a nonmagnetic material and a fixed layer made of a ferromagnetic material and of which the magnetization direction is fixed are laminated in that order and in which information is recorded by the use of a change of a magnetization direction of the free layer, the magnetoresistive effect element is characterized in that the free layer is divided into a plurality of regions, a plurality of regions are located around a write electrode extending in the lamination direction of each layer so as to surround the write electrode and the respective regions surrounding the write electrode constitute a magnetic field returning structure.
According to a further aspect of the present invention, there is provided a magnetic memory device including a magnetoresistive effect element in which at least a free layer made of a ferromagnetic material, a nonmagnetic layer made of a nonmagnetic material and a fixed layer made of a ferromagnetic material and of which the magnetization direction is fixed are laminated in that order and in which information is recorded by the use of a change of the magnetization direction of the free layer. The magnetic memory device includes a magnetic field returning structure for returning a magnetic field generated by the free layer.
In accordance with yet a further aspect of the present invention, there is provided a magnetic memory device including a magnetoresistive effect element in which at least a free layer made of a ferromagnetic material, a nonmagnetic layer made of a nonmagnetic material and a fixed layer made of a ferromagnetic material and of which the magnetization direction is fixed are laminated in that order and in which information is recorded by the use of a change of a magnetization direction of the free layer. This magnetic memory device is characterized in that the free layer is divided into a plurality of regions, a plurality of regions are located around a write electrode extending in the lamination direction of respective layers so as to surround the write electrode and the respective regions surrounding the write electrode constitute a magnetic field returning structure.
According to the magnetoresistive effect element and the magnetic memory device having the above-mentioned arrangement, since the magnetoresistive effect element and the magnetic memory device have the magnetic field returning structure, a magnetic field, generated by the free layer of the magnetoresistive effect element, is returned so that a magnetic field can be prevented from being leaked to the outside of the magnetoresistive effect element as much as possible. Accordingly, even when the magnetoresistive effect element is further microminiaturized and is increased in integration degree, the adjacent magnetoresistive effect element can be prevented from being influenced by a leakage magnetic field. Further, since a magnetic field is returned, a magnetic moment can take a uniform state within the free layer of the magnetoresistive effect element, whereby magnetic coercive force can be stabilized.